This invention is related to steelmaking, particularly, but not exclusively to the production of ultra low carbon steel in a tank degasser.
In recent years the demand for ultra low carbon steel (carbon content less than 40 parts per million) has been increasing. The production of this steel necessitates the use of vacuum treatment, either by the Ruhrstahl-Heraeus (hereafter RH) vacuum process, the Dortmond-Horder Huttenunion (hereafter DH) process, or the tank degassing process. The tank-degassing route has the advantage of low investment and low inert gas consumption. However, the vacuum decarburization process in a tank degasser is slower than in a RH degasser, due to limited space in which liquid steel can freely flow and splash, resulting in smaller reaction surface area. This feature extends the treatment time, reduces heat size, results in higher heat loss of liquid steel, raises the energy consumption, and reduces the capability of carbon removal. Therefore, it is desirable to improve this process.
In traditional vacuum tank degassing, a ladle holding liquid steel is placed in a vacuum chamber. Gas injectors, usually porous plugs, are installed in the ladle bottom to stir the steel. This gas injection stirring configuration is derived from conventional ladle refining process under atmospheric pressure, usually designed on the basis of liquid metal mixing time and/or top slag -metal reaction rate measurement. As such the special conditions and gas state changes associated with vacuum degassing have not been considered.
With such conventional arrangements, the plugs are positioned so that when, under vacuum, inert gas is injected from these plugs, two bubble plumes are formed bringing liquid steel flow upward. The upward velocity of steel makes the slag layer at the top separate, and form two open areas of bare steel. The circulation pattern of liquid steel is such that when steel reaches the top surface, it flows outwards along the surface, then turns downwards, and flows back to the bottom in a toroidal pattern. For most of the decarburization time, the main decarburization site is the two open areas, which is a function of gas injection rate and slag layer thickness. Limited gas injection rate produces limited open areas and limited liquid phase turbulence, preventing the enhancement of the decarburization rate. Moreover, the interaction between the two plumes creates two circulation loops in the bath, producing resistance to the liquid circulation, and reducing the renewal rate of liquid steel at the top surface.
In the past there have been some trials on side injection in steel ladles, for example, powder injection into liquid steel for desulfurization, developed by injectall. This device resembled a revolver with a rotating magazine on the outside of the ladle wall. When injector clogging occurred, it was rotated to the next injector. SAFE Company, in France, tested side injectors in ladles about 20 years ago, to avoid the bottom injector clogging by slag at the end of a cast, All these tests were carried out under atmospheric pressure.
Side injection under atmospheric pressure has a number of disadvantags. Steel in ladles is usually deoxidized with aluminum and covered by a layer of slag. To avoid oxidation by air, surface exposure due to gas blowing up the slag layer should be minimized. This necessitates a limited gas injection rate to prevent excessive surface exposure of liquid steel. Ladle refining processes are aimed at homogenizing liquid composition and temperature, enhancing alloying, and/or promoting slag-metal reactions. These procedures require the maximum mixing effect from the injected gas. Side injected gas bubbles travel less distance than those from bottom injectors, providing less liquid pumping power. Moreover, sidewall refractory erosion may increase. Therefore, side injection is considered undesirable for conventional ladle refining processes, and not practiced.
In the production of ultra low carbon steel in tank degassing, the bottom injected gas flow rate is more than enough to provide adequate mixing due to gas volume expansion. However, the liquid splashing, produced by large bubbles breaking at the bath surface, and causing steel loss and operation problems, is a major concern, and a limiting factor for further increase in gas injection rate. This is unlike the situation in a RH device, where high gas injection rate, up to 620 Nm3/hour for a 260 tonne capacity device, can be used to create vigorous liquid splashing to increase the surface area of liquid metal exposed to vacuum. For a tank degasser of similar capacity, the upper limit is less than 50 Nm3/hour. Therefore, the surface area exposed to vacuum in a tank is much more limited. The presence of slag in the ladle further limits the exposed surface area. Furthermore, the turbulence intensity of the liquid steel is much lower, due to less stirring energy. These are two of the key factors limiting the carbon removing capacity of a tank degasser compared to a RH degasser.
The research work of S. Kitamura et al, published in Tetsu-to-Hagane, Vol. 80(1994), No.2, pp 13-18, in a vacuum induction furnace, shows that the active surface area of liquid metal, agitated by breaking bubbles, is of primary importance for the reaction between liquid metal and gas. As vacuum decarburization is a liquid-gas reaction, an increase in the number of inert gas bubbles released from the bath will enhance the reaction. However, such side injection does not promote effectively the circulation of steel within the ladle.
It is therefore an object of the present invention to obviate or mitigate the above disadvantages in a manner that permits an increase in the gas injection rate of a steel ladle while keeping the steel splashing at an acceptable level.
According to one aspect of the present invention there is provided a method of promoting the circulation of liquid steel within a ladle having a base and a sidewall upstanding from the base, the method comprising the steps of locating a plurality of gas injectors asymmetrically in said base of said ladle and at positions relative to one another to promote a common circulatory flow path of steel in said ladle, and injecting gas through the injectors.
According to a further aspect of the invention there is provided a ladle comprising abase, sidewalls extending upwardly from the base, a plurality of injectors in the base positioned asymmetrically on the base and located relative to one another to promote a common circulatory flow path of steel in the ladle.
According to a still further aspect of the invention there is provided a method of promoting circulation of liquid steel within a ladle comprising the steps of injecting a gaseous stream into the liquid steel to promote a common circulatory flow upwardly along one side of said ladle and downwardly along another side of said ladle.
In general terms, a preferred embodiment of the invention comprises a ladle, equipped with both bottom gas injectors and side nozzles, containing liquid steel to be refined, and placed in a vacuum tank. Inert gas is injected into liquid steel during the treatment. Gas injected from the bottom is designed to provide sufficient liquid circulation, and slag free surface at the top of the bath. Injection from the side is for the creation of large quantity of smaller bubbles, helping to enhance the liquid circulation, enlarge the top slag-free surface, and increase the activity at the top surface.
Because the static pressure at the side injection site is much lower than that at the bottom under vacuum, the actual gas volume injected from the side is correspondingly expanded, producing more bubbles than under atmospheric pressure.